/*
 * Copyright 2002-2017 the original author or authors.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package org.springframework.web.servlet.resource;

import java.util.List;

import javax.servlet.http.HttpServletRequest;

import org.webjars.WebJarAssetLocator;

import org.springframework.core.io.Resource;
import org.springframework.lang.Nullable;

/**
 * A {@code ResourceResolver} that delegates to the chain to locate a resource and then
 * attempts to find a matching versioned resource contained in a WebJar JAR file.
 *
 * <p>This allows WebJars.org users to write version agnostic paths in their templates,
 * like {@code <script src="/jquery/jquery.min.js"/>}.
 * This path will be resolved to the unique version {@code <script src="/jquery/1.2.0/jquery.min.js"/>},
 * which is a better fit for HTTP caching and version management in applications.
 *
 * <p>This also resolves resources for version agnostic HTTP requests {@code "GET /jquery/jquery.min.js"}.
 *
 * <p>This resolver requires the "org.webjars:webjars-locator" library on classpath,
 * and is automatically registered if that library is present.
 *
 * @author Brian Clozel
 * @see org.springframework.web.servlet.config.annotation.ResourceChainRegistration
 * @see <a href="http://www.webjars.org">webjars.org</a>
 * @since 4.2
 */
public class WebJarsResourceResolver extends AbstractResourceResolver {

    private final static String WEBJARS_LOCATION = "META-INF/resources/webjars/";

    private final static int WEBJARS_LOCATION_LENGTH = WEBJARS_LOCATION.length();


    private final WebJarAssetLocator webJarAssetLocator;


    /**
     * Create a {@code WebJarsResourceResolver} with a default {@code WebJarAssetLocator} instance.
     */
    public WebJarsResourceResolver() {
        this(new WebJarAssetLocator());
    }

    /**
     * Create a {@code WebJarsResourceResolver} with a custom {@code WebJarAssetLocator} instance,
     * e.g. with a custom index.
     *
     * @since 4.3
     */
    public WebJarsResourceResolver(WebJarAssetLocator webJarAssetLocator) {
        this.webJarAssetLocator = webJarAssetLocator;
    }


    @Override
    protected Resource resolveResourceInternal(@Nullable HttpServletRequest request, String requestPath,
            List<? extends Resource> locations, ResourceResolverChain chain) {

        Resource resolved = chain.resolveResource(request, requestPath, locations);
        if (resolved == null) {
            String webJarResourcePath = findWebJarResourcePath(requestPath);
            if (webJarResourcePath != null) {
                return chain.resolveResource(request, webJarResourcePath, locations);
            }
        }
        return resolved;
    }

    @Override
    protected String resolveUrlPathInternal(String resourceUrlPath,
            List<? extends Resource> locations, ResourceResolverChain chain) {

        String path = chain.resolveUrlPath(resourceUrlPath, locations);
        if (path == null) {
            String webJarResourcePath = findWebJarResourcePath(resourceUrlPath);
            if (webJarResourcePath != null) {
                return chain.resolveUrlPath(webJarResourcePath, locations);
            }
        }
        return path;
    }

    @Nullable
    protected String findWebJarResourcePath(String path) {
        int startOffset = (path.startsWith("/") ? 1 : 0);
        int endOffset = path.indexOf("/", 1);
        if (endOffset != -1) {
            String webjar = path.substring(startOffset, endOffset);
            String partialPath = path.substring(endOffset + 1);
            String webJarPath = webJarAssetLocator.getFullPathExact(webjar, partialPath);
            if (webJarPath != null) {
                return webJarPath.substring(WEBJARS_LOCATION_LENGTH);
            }
        }
        return null;
    }

}
